Electronic control apparatus



W. H. GILLE E L 1 .u r RG m A w m H W. q VB uw T S NL T. .t IL au w M h s 2 s, Y M nD R m mm n .L O m |1||| w c (Nm w. (n: N O Dn T m 2 2, m2 2 nv u J w n a fred ha 1 R1, 955,5 m n LEES 1 l l I l I I I I I l I l l l I I l s I.. a .w zgba .n s O Dec. l, 1953 w. H. GILLE 2,651,449

ELECTRONIC CONTROL APPARATUS Original Filed June 22, 1942 Sneets-sheet 2 E. 2 i//B l "5 in..

INPUT VOLTAGE INVENTOR. WLUS H. GLLE.

Patented Dec. 1, 1953 UNITED SKATE@A Ztlgifi OFFICE ELECTRONIC CGNTROL APPARATUS Willis H. Gille, St. Paul, ll/linn., assigner to Minneapols-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware (Cl. 31S--28) 14 Claims. l

This application is a division of my copending application No. 447,989, led June 22, 1942, and is assigned to the assignee of the present applin cation.

The present invention relates to electronic ainpliers and particularly to amplifiers adapted for use in control systems of the type wherein a device is controlled. in accordance with an elec trical signal produced by means responsive to a controlling condition.

An object ci the present invention to provide an improved electronic amplifier circuit whose output is selectively intermittent or continuous, depending on the magnitude of the input signal.

A further object of the present invention to provide an improved amplifier circuit for `controlling the operation of a relay in accordance with a variable condition.

A further object oil the present invention is to provide an electronic amplifier circuit having improved anti-hunt characteristics.

A further object of the present invention is to provide an improved electronic ampliiier circuit for operating either of two relays in the output circuit, depending upon the phase of the input signal.

A. further object of the present invention is to provide an improved electronic amplifier circuit in which weak signals are aided in operating a relay in the output circuit.

Other ohiects advantages of the present invention will hecoine apparent :from a considera tion of the appended specification, claims, and drawings, in which:

Figure l is circuit diagram or an ainplier embodying the principles of the invention shown in control syste-ni wherein it may 'ce used adf vantageously,

Figure, 2 a graphical illustration of the operation of a portion oi the circuit of Figure l., and.

ure 3 is a portion oi the circuit of Figure l, redrawn so that the relationship of the circuit elements to each other be incre clearly understood.

describing the amplifier per se, a 1:rief description wi he given ci apparatus with which the amplier is designed to cooperate.

There is shown Figure l, somewhat diagrammatically, a system for controlling the rudder oi aircraft. Referring to thin igure, there is shown a direction-responsive device it which ptetl rotate a shaft li accordance with the from a course of an aircraft i ch the device lil mounted. The device lll may he a compass of either the nia-gable direction-responsive device. For the pure poses o the present illustration, it is indicated by 'way oi example, that the device l@ is a directional gyroscope.

Fixed on the shaft il for rotation therewith is a slider l2 which cooperates with a slidewire resistance li. The slider l2 and the slidewire resistance it together forni a potentiometer generally indicated at lil, hereinafter referred to as the rudder control potentiometer. The potentiometer lil forms part of a balanced circuit genindicatecl at l5, and hereinafter referred to as the rudder control network. The rudder control network l5 is a balanced circuit generally of the Wheatstone bridge type, and includes input terminals it and il, and output terminals i3 and i9. Output terminal i8 is in electrical connection with slider lll, and output terminal i9 is in. electrical connection with a slider 22 which cooperates :with a slidewire resistance 23. Slider 22 and slidewire t3 together form a potentiometer hereinafter referred to as the rudder reioalancing potentioineter.V input terminals it and il of rudder control network l5 are connected through conductors 25 and 25, respectively, to power supply terminals A and A'.

The network l5 includes the customary four branches of a Wheatstone 1oridge circuit connecting each of the input terminals with each of the output terminals. The upper leitbranch of network l5 connects input terminal and output terrninal iS, and may be traced from input terminal I5 through a variable resistance 2l, a conductor and the left hand part of slidewire resistance to slider l2 and output terminal i8. The upper right hrai'ich of the network l5 connects input terminal il `with output terminal i5 and may be traced from input terminal il through a variable resistance Si?, a conductor 3l, the right hand. section ci slidewire resistance to slider i2 and output terminal l t. The lower left branch of network connects input terminal lli with output terminal lil and may he traced from input terminal it through a conductor 32 and the left hand. section of slidewire resistance to if er 22 and output terminal rihe lower t 'oran-:h ci network connects input ter- 1 il with output terminal il? and may he traced from ,input terminal il through. a conductor 33 and the right hand section oi slidewire resistance to slider 22 output terminal lil.

The variable resistance devices il' and here iter referred to as the rudder centering .adjustment rheostats.

A variable resistance device 34 is connected in para el with the slidewire resistance i3. The resistance de ce 34 .is hereinafter referred to as the rudder i o adjusting rheostat.

Output terminals I8 and I9 of rudder control network l are connected through conductors 35 and 36, respectively, to input terminals 31 and 33 of an amplifier and power supply unit 43, which, for the sake of simplicity, will be hereinafter referred to during the description of the apparatus with which the amplier cooperates as amplifier 46.

The amplifier 4I), which is described in detail hereinafter, operates to sclectivelycontrol'the energization oi a pair of output terminals 4 I and 43. Output terminals 4I and 43 are connected by conductors and 5 I, respectively, to terminals 41 and 53 on a power transmission unit indicated schematically at 53. The power transmission unit 5I) is provided with a pair of grounded terminals 48 and 54.

Power transmission unit 56 is provided with a power input shaft and a power output shaft 56. Power input shaft 55 is continuously rotated in one direction by a direct current shunt motor 51, having an armature and a field winding 6I. The terminals or" armature 6I] and field winding 6I are connected through conductors 62 and 63 to the terminals of a suitable source of electrical energy, shown as a battery 64. YA switch including a switch arm which is operable vinto engagement with a stationar5T contact 66 is connected in the conductor V62, so that the motor 51 may be deenergized. The conductor 63 is connected to ground at l69, thereby grounding one terminal of battery 64.

The amplifier 40 is provided with power input terminals 1D and 1I. Terminal 10 is connected `through a conductor 12 tothe grounded terminal of battery 64. Power input terminal 1I is connected through a conductor 13,1in which a switch comprising a .stationary contact 14 Yand a movable contact arm 151s inserted, 4to conductor 62, and thence to the opposite terminal of battery 64.

VAmplifier V4t is also provided with a pair of power output terminals 16 and 11, which are connected through suitable conductors to power supply terminals A, A of the rudder control ynetwork I5.

There is mounted on power output shaft 56 of the servoniotor unit 5G, a pulley iBU over which passes a cable 6 I, the ends of which are connected to suitable mechanism for operating the rudder of the aircraft on which the system is mounted. The slider t2 of the rudder rebalancing potentiometer 24 is also :fixed on the shaft 56 to rotate therewith.

Operation of .rudder control in Figure 1 When the parts are in the positions shown in the drawing, the control slider I2 and the follow up slider 22 are engaging the mid-points of their respective slidewires. Furthermore, the centering rheostats 21 and 30 have equal settings. Uni der these conditions, the electrical potentials of the output terminals I8 and I9 of network I5 are both equal to the median potential of the source of energy connected. to input terminals I6 and I1. Since there is no potential difference between output terminals 18 and I9, no signal is applied to the signal input terminals 31 and 38 of amplifier 40.

The amplifier 40 operates, as more fully de scribed hereinafter, in such a manner that when As previously described, the power input shalt.

55 of power transmission unit 56 is continuousli" rotated by the motor 51. Power transmission unit .50 contains .a gearing arrangement, driven by motor 51, which operates through a pair of clutching members selectively energized by the ampliiier V4I) to control the direction of rotation of output shaft '56 depending upon the clutching member `that vis energized. When no current flows either Vin the circuit including terminals 41 and 48 or in the circuit including terminals 53 and 54, the power output shaft 56 is stationary. When the circuit including terminals 41 and it is energized, or when the circuit including terminals 53 and 54 is energized, a connection is completed between power input shaft 55 and power output shaft 5,6, such that the shaft 56 is rotated in one direction or the other, selectively, depending upon which of the two circuits mentioned is energized. A more complete description of power transmission unit .56 and its operation is to be found in my parent application 4l7.989, iiled June 22, 1942.

summarizing, it may be stated that the operation of amplifier 4I) and power transmission unit 50 are such that when an alternating potential of the same phase as that applied to input terminals I6 and I1 of network I5 is impressed on the signal input terminals 31 and 33, the shaft 56 is rotated in a counter-clockwise direction so as to move slider .22 to the left along slide wire resistance 23. On the other hand, when an alternating signal of phase opposite to that of the potential supplied to input terminals I6 and i1 is impressed on signal input terminals 31 and 33, the power output shaft 56 is rotated in a clock wise direction, thereby moving slider 22 to the right along .slidewire `resistance 23. When no signal is `impressed on signal input terminals 31 f and 438, the shaft 56 remains stationary.

As long as the aircraft stays on the course which the directional gyroscope I0 has been set to maintain, the slider I2 is held stationary in the center of the slidewire resistance I3. When `the aircraft deviates from the established course, the directional gyro I0 moves the slider I2 along the resistance I3. For example, let it be assumed that the aircraft deviates from its course in such a direction that the ,slider I2 is moved to the lelt along the slidewire I3. When this movement takes place, it will be seen that the potential of output terminal I8 is changed to a value closer to that of input terminal I6 than `that of input .terminal I1. There is then a potential diierencc between output terminals I8 and I9, and the phase of this potential diilerence is the same as that of the voltage applied to input terminals I6 and I1. This potential cliiierence is impressed as a signal on the signal input terminals 31 35 of amplifier 46. As previously described, a potential of this phase impressed on the terminals 31 and 38 acts through the amplier 40 and the power transmission `unit 50 to cause rotation of power youtput shaft 56 in a counter-clockwise direction. This rotation of shaft 56 acts through pulley and cables 8| to apply a corrective deflection to the rudder (not shown) so as to restore the aircraft to its predetermined course. At the `same time, the slider 22 is moved to the lett along slidewire resistance 2s. When the slider 22 reaches a position such that the potential of output `terminal ifi is equal to new potential oi output terminal i3, no furthi signal is applied to thD aniplier signal input terminals and and rotation of shaft 55 ceases.

Because or the corrective deflection of the rudder, `the aircraft' returns to its established course. the craft returns to its established course, .he directional gyroscope il) Ymoves the slider l?. to the right along resistance 23, back towards its center position. As this motion takes place, it will be seen that the potential Iof out- ;ont term' `l becomes closer to that of input terminal if, than that of the output terminal Iii. Therefore a J between output terminals I9, but time phase opposite to that oi the voltage iinpressed on the input terminals It and il. This potential d'tierence is impressed on the amplifier signal ir t terminals I 33, and acts through ai and servoinotor unit s@ to cause rotation of in a clockwise 4direction. rotation shaft acts through pulley and cable to restore the rudder to its orrnal position, and also moves the slider `22 to ne right along slidewire resistance 23. When the slide` reaches the position shown on the drawings, no potential diierenoe exists between output `terminals is and l, and the motion oi shaft `lift is stopped.

It will he readily understood foy those skilled in the art that if the aircraft devia/tes from its course in such a direction as to move slider l2 to the right, the system responds in a i anner entirely analogous to that previously de. ribed. The shalt "e nrst rotated clockwise to apply a corrective deflection to the rudder and to move the slider 22 to the right along slidewire resistance so as to rehalance the network I5. After the aircraft has returned to its established course because oi the deflection oi the rudder, the slider i2 is moved cach to its center position by the directional gyroscope id, and the system responds to drive shaft counter-clockwise to remove the corrective deflection from the rudder and to drive slider to the left back to the center position shown on the drawing. Operation of rheostats 2l and changes the position of slider 22 with respect to slidewire 223 .at which the network iii is balanced. Wor example, assume that rheostat 2l is or ated so that its resistance is increased and vthat the resistance of rheostat 3@ is decreased. it will be readily understood that by suoli operation the potential at the slider I2 is rr e to approach closer to that of the input terminal il. s. potential diierence then exists between output terminals is and i@ which is opposite in phase to the potential applied to the input terminals le and il'. This unbalance potential is applied the signal input terminals and Elli of ampliiier llt, thereby causing shaft to be rotated clockwise so as to move slider 22 to the right along resistance 23 and rebalance network lli. it will be readily understood that this produces a deflection of the rudder which is maintained as long as the rheostats 2l t@ remain in their adjusted positions.

The ratio adjusting rheostat Sli operates to determine the amount of movement of the rebalancing slider 22 necessary to rebalance the network i5 after a given movement of control slider I2. It will be readily understood that rheostat 34, by changing the resistance connected in parallel with slidewire resistance I3, changes the total vdifference of potential between the terminals of the slidewire resistance I3. When the total potential difference across resistance lI3 is changed, the resistance drop per unit length of resistance I3 is also changed. The resistance drop per unit length of the slidewire resistance however, remains constant. When slider i2 moves through a dista-nce corresponding to a certain potential difference along the slidewire resistance I3, `the slider 22 must move along the resistance 23 a sucient distance to cover the same potential difference in order to reha-lance the network Iii. It may therefore be readily seen that if the potential change per unit length of resistance i3 is changed, the distance which the slider 22 must move to rebalance a given movement oi slider I2 is also changed.

Having described the apparatus with which the alipliiier of the present invention is do he employed, the ampi ...er itself will now be dem scribed.

The terminals 'lo and "il which are supplied with electrical energy the battery ifl of iigure l are connected to a vibrator inverter oircuit oi conventional type. The vibrator coinprises a coil IGI which controls the movement of a switch a in i532 between stationarv contacts H33 and its. condenser is connected between the stationary contacts i533 loll in order to reduce sparking at those contacts. A conductor it connects terminal l@ with the switch arm itt and with one terminal oi" coil itl. The opposite terminal of coil iti is connected hy a conie'i to the upper terminal oi a primary i -riding of a power supply transformer ilo. The conductor itl' also connected to the stationary contact lili?. Terminal li is connected by a conductor Ill to mid-point tap H2 on the transformer primary winding iil. lower terminal of primary win ig lil is connected through a conductor ilo to stationary contact liifl. A filter condenser Elfi connects stationary Contact Iili with grounded 'terminal lil. uanother filter condenser H connects mid-point tap H2 with switch arro ISE.

The operation of the inverter circuit its?, which will be readily understood those skilled in the art, may be briefly outlined follows:

When the switch arm ist engages Contact ltd, as shown in the drawing, a circuit is completed which may be traced from power input terminal l@ through conductor switch arin lo?, ccntact Il, conductor il-Ei, the lower half oi priinary winding lofi, mid-point il?, and conduc-tor iii to battery terminal li. it the time, a circuit is completed for ener` iii! which may he traced. from tern inal 'iii through conductor coil lili, conductor the upper half of primary winding raid-point tap and conductor iii to terminal li. The current flowing through this second circuit will be much lower than that through the rst circuit traced on account of the impedance of coil i il i. The net flux in the ransiorrner core therefore has a direction determined by the direction or current now in the lower half of winding i. The energization of coil lili causes switch arm ist to move upwardly into engagement with Contact its. This opens the circuit through the lower half of primary winding l llt, and shunts coil lill so as to increase the current flowing in the upper half of primary windips |08- The direction or .flux flowing Dil the transformer is then determined by the direction of current ow in .the upper half `oji primary 'winding |06, being the opposite -to vthe direction of current flow occurring when the switch anni |02 engages contact |64. Since the coil |0| has now been shunted, the switch arm |62 drops back to the lower contact |64, to which position it is biased by means not shown. thereby again caus.- ing the current iow in the lower haii of primari winding |63 to predominate and again reversing the direction ol" flux dow in the transformer core. This sequence of events repeats itself cyclically, thereby producing an alternating linx in the transformer core H6, and inducing alternating electrical potentials in secondary windings ||6 ||1 with which transformer i is provided.

The secondary winding ||1 is the power supply winding for the bridge l5.

The secondary winding I|6 supplies power to the ampliler circuit 40. The amplifier 40 comprises two preliminary voltage amplification stages |26 and |2| and a final power stage' |22. A rectifier triode |23 is used to supply power to the voltage amplication stages and |2I, and biasing potential i'or one or" those stages. Another rectifier triode 24 is used to supply biasing potential for the nnal output stage |22.

Signals applied to 'the input terminals 31 and 33 of amplifier 40 are impressed across a potenti'- ometer resistance |25, one terminal of which is grounded as at |26. An adjustable tap |21 movable along the resistance |25 controls the gain of the amplifier 40. tion stage |20 oi amplier 40 includes a triode having an anode I3|, a control electrode |32, a cathode |36, and a heater element |34, which may be supplied with electrical energy from any suitable source (not shown). The input circuit of the first stage |20 may be traced. from control electrode |32 through a conductor |35, tap |21, resistance |25, and ground connections |26 and |36 to cathode |33. It should he noted that this input circuit is unbiased. v

The output circuits of the stages |20 and |2| are supplied with electrical energy from a rectifier circuit connected to the upper half of transformer secondary winding H6. This rectiiier circuit may be traced from the upper terminal or secondary winding ||6 through conductors |31, and |40, anode |4l, and cathode |42 of a triode |23, a resistance |43, a conductor |44, a fixed resistance B, a potentiometer resistance |45, ground connections |46 and |41, a conductor |48, a resistance |56 in parallel with a condenser |5|, and a conductor |52 to a mid-point tap |53 on transformer secondary winding |6. The triode |23 provided with a control electrode |54 which. connected to the cathode to maintain the triode in a highly conductive condition. Filter condcnsers |55 and |56 are connected between the opposite terminals of resistor |43 and ground in order to by-pass high irequency components appearing in the output ol" the rectier circuit. The conductor |44 serves as the positive terminal of the power supply for the amplllication stages |20 and |2|, and ground serves as the negative terminal. The direction of current flow through this rectifier circuit is such that the potential drop across resistance renders its right-hand terminal positive with respect to its left-hand terminal, as indicated yby the legend in the drawing.

The output circuit of the first stage |20 may The lirst voltage amplicab e traced, from positive power .supply terminal |44 through a conductor |51, a load resistance |58, anode |3I, and cathode |33, to ground at |36.

The second voltage amplification stage I2| inoludes a triode |60 having an Vanode |6|, a control electrode |62, a cathode |63 and a heater element |64. Heater element |64 may be energized from any suitable source of electrical energy v(not ShOWIU The output circuit of stage |20 is coupled to the control electrode |62 of stage |2| through a blocking condenser |65, a protective resistance |66, and alprotective resistance |61.

The input circuit of stage |2| is divided into two branches. The first branch may be traced from control electrode |62, through protective resistance |61, nresistance |68, variable resistance |10, and a by-pass condenser |1| in parallel, a conductor |12, a movable tap |13 associated with variable resistance |45, and ground connections |46 and |14 to cathode |63. The second branch of the input circuit of stage |2|, hereinafter termed the feedback circuit, may be traced from control electrode |62 through resistances |61 and |68, a conductor |15, a condenser |16, a conductor |11, resistance |50 in parallel with by-pass condenser |5I, conductor |48 and ground connections |41 and |14 to cathode |63. The output circuit of stage |2| may be traced from conductor |44 which serves `as the positive power supply terminal, through a load resistance |18, anode |6|, and cathode |63 to ground at |14. which is the negative terminal of the power supply.

The final output stage |22 includes a twin tetrode |50, whose individual tetrodes IBI and |02 are oppositely connected to the upper and lower halves of secondary winding ||6, respectively. The tetrode |il| includes an anode |63, a first control electrodo or screen grid |64, a second control ,electrode |85, a cathode |86, and a heater filament |61. The tetrode |62 includes an anode |90, control electrodes |9| and |62, a cathode |63, and a heater lament |94. The heater filaments |81 and |94 may be connected to any suitable source of electrical energy (not shown).

A bias potential for the main control electrodes |85 and |92 of the final stage |22 is supplied from a circuit which includes the rectifier triode |24. Triode |24 has an anode |95, a control electrode |96, a cathode |91, and a heater lament |98. The heater filament |96 may be energized from any suitable source of energy. Control electrode |96 is connected to cathode |91 by a conductor 200, so as to maintain triode |24 in a highly conductive condition whenever a sufficiently high potential of proper polarity is applied `to the cathode and anode. The circuit through the rectifier triode |24 may be traced from the midpoint ,tap |53 of transformer Secondary Winding I6 through conductor |52, resistance |50 and its parallel by-pass condenser |5|, conductor |48, ground connections |41 and 20|, fixed resistance 92, a potentiometer resistance v2 02, a lixed resistance 9|, anode |65, cathode |91, resistance 203, and conductor 264 to the lower terminal of secondary winding H6.

It should be noted in passing ythat the current flow through the rectifier circuit including triode |23 passes through resistance |50 and its parallel by-pass condenser |5| in a direction opposite to the Current flow through rectifier mode 124. S1110@ thess .two currents are substantially equal and QPPOSLE, and .Since they .both Ilow during the geen@ by means o movable tap associated with resistance The direction or current iiow through triolde 2d is such as to make the lefthand terminal oi resistance 202 positive with respect to its right-hand terminal, as indicated by the legend inthe drawing.

The tet-redes i! and E02 of the inal output stage E22 have a coinnion input circuit, which may he traced from control electrodes 05 and |02 to conductor resistance 200, a conductor 2530, condenser and ground connections 20| and 01 'to cathodes and iet. The output circuit or' the second preliminary amplification stage iii is co1 vi to the cornrnon input circuit oi the te des anc. 02 through a blocking condenser i connected between anode i6! and conducto The output circuit oi tetrode 10i may be traced from the upper .terniinal of transformer seconc rv v' ng ii through conductor 631, relay Win et condenser 21g in parallel, a onductor anode ift, cathode |00, conductor denser lili, and conductor to mid-point tap its on transfwmer secondary winding liti.

The outpu'L cuit of tetrode iii?. may be traced .troni the lower end of transformer secondary winding tit through conductor relay Winding 017i and condenser if in parallel, the conductor itf, anode i cathode 103, conductor ist, resistance E50 and its parallel by-pass condenser isi, and conductor E52 to mid-point tap 53 on transformer secondary winding l i0.

The control electrode its of tetrode iti, which is conventionally termed a screen grid, is oonnected through resistance 2 i5 and conductors 2i?, i3?! and lill? to the upper terminal of secondary winding Sie. Control electrode 0| of tetrode 102 is connected through a resistance EIS and conductors 220, and 201i to the lower terminal of secondary winding H6. A group oi three buffer cendensers 22, 223

24 in series is connected across the terminals of econdary winding iiii by means of conductors E31, i30, 22! and 2061.

The following table shows, by way of example, values of resistance and capacitance for the various circuit elements which have been used in one embodiment of the amplifier circuit of Figure 1:

Reference character of element:

and

Electrical quantity i -microfarads-- 1 i it do 1/4 iti do 25 i253 ohnis 10,000 it-3 do 5,000 ifi-5 do 50,000 |50 do 800 i5@ microfarads 25 olo 50 do 10 l niegohrn :Ar its microiarad-- .05 ido inegohm 1/2 'i do 1/2 00 do 1/2 |10 do 1/2 i1 microfarad-- .1

resistance and its parallel oy-pass con- |16 do .04 |18 megohm 1A 202 do .1 203 l do 1/4 205 imicrofarads 8 208 "ohms, 150,000 2 l rnicrofarad .05 2 l 2 do 1 2 i4 e do 1 2 IS ohms 10,000 2|8 do 10,000 L22 microfarad .007 22s do .007 22s do .007

90 megohrn 1A.;

0| ohms 20,000

02 megohrn-- 1 The triodes and may be included in a, twin triode tube. For example, a type 71"*7 tube has been found satisfactory. The twin tetrode |80 may be of the type 6V5GT. The triodes 623 and 24 may be the two parte of another type 'TF7 tube.

When an alternating signal potential appears at the input terminals 31 and of anipliner e0, it is amplified in a conventional manner by the irst preliminary stage |20, and the amplified signal is transmitted through blocking condenser and is applied to the input circuit of the second voltage amplification stage |26.

The second amplification stage l2! has a peculier input voltage-output current characteristic which may be better understood by reference to Figures 2 and 3. In Figure 2, the curve A represents the grid voltage-anode current characteristic of the triode 150, and it should be noted that the curve A is of a generally conventional form. Referring to Figure 3, it will loe seen that a high resistance |61 (1/2 megohin) is connected in series with the control electrode 52, and that the control electrode |62 is positively biased by its connection through resistances it?, ist and 510 and conductor i12 to the tap 51e which is movable along potentiometer resistance its. The resistance 05 is connected between the positive power supply line iM and ground at itt. Since the cathode |03 is grounded at ils, it will be seen that all points on the resistance his are positive with respect to the cathode Therefore a current low takes place through the connection from the tap |15 through conductor i12 and resistances |10, |68 and |01, control electrode E02. cathode |63, and ground i115. The potential drop due to this current now may be considered as divided into two portions, one portion being the drop across the resistances i0?, |00 and 110, and the other portion being the potential between the control electrode |62 and the cathode Since the impedance of the resistances and is very high as compared to the control electrodeto-cathode impedance o1 the triode the voltn age drop across the resistanoes is very inuch greater than the control electrode to cathode potential. and the latter potential is in fact very small.

Considering this circuit still further, it will oe apparent that if the Control electrode to cathode voltage tends to increase, the resulting increase in current flow causes the voltage drop across the resistances |61, itt and il@ to incerase in proportion. This increase in the potential drop across the resistances tends to restore the control electrode to the same potential which it orig-1i nally had. It may therefore seen that the potential of the control electrode U52 is maintained at a value only slightly positive with respect to the cathode |63, and that it is not possible to increase the control electrode potential substantially in a positive direction by applying an additional positive potential to the circuit just traced, or to any point on that circuit to the left of the resistance |61.

Because of this interaction of the potential drop across resistance |61 and the positive bias potential obtained from the tap |13, the input voltage-output current characteristics of the stage |2| as a whole somewhat "-nt the input voltage-output current characteristics of the triode |60 as shown at A in Figure 2. In the present discussion of the input voltage-output current characteristic of the stage |2|, the input voltage is intended to signify the potential between the cathode |63 and the terminal of resistance |61 which is opposite to that terminal connected to control electrode |52. In other words, the input voltage is the potential between the left-hand terminal of resistance |61, as it appears in the drawing, and the cathode |53.

Referring now to Figure 2, it may be seen, that as a result of the action of the potential drop across resistance |61, described above, when the input voltage of the stage |2| increases from a negative value in a positive direction, the output current increases in the manner indicated by the characteristic A until the input voltage reaches a slightly positive value. After such a slightly positive value of input voltage has been reached, the potential drop across resistance |61 comes into play to maintain the control electrode potential at a substantially constant value, and hence the output current remains at a substantially constant value also. Therefore the characteristic of the stage |2| does not follow the curve A after the input voltage becomes positive, but instead follows the straight line B, indicating that the output current of the stage |2| remains substantially constant for all values of input voltage greater than a small positive value. In obtaining the characteristics A and B experimentally, it has been found that the line B makes a sharp angle with the curve A.

Any tendency of the output current of trode |60 to change is further reduced by the action of the resistance |18. It will be understood that when an increase in the current ow through the resistance |18 takes place, the voltage drop across resistance |18 increases correspondingly, thereby reducing the anode to cathode voltage applied to the triode |60. This results from the fact that the voltage supplied between the line |44 and ground is divided between the drop across resistance |18 and the drop between the anode |6| and cathode IGS. The decrease in anode to cathode voltage applied to triode |60 resulting from an increase in current flow through the resistance |18, itself causes a reduction in the current iiow through the anode-cathode circuit of the triode |60 thereby tending to restore the current flowing through this circuit to its previous value.

Now consider the operation of the stage |2|, having the characteristics described above, in the amplifier circuit shown in Figure l. Referring to Figure 2 as an illustration of this operation, let it be assumed that the adjustable tap |13 is set so that the potential drop across resistance |61 has a value indicated at B in Figure 2, so that the left-hand terminal of resistance |61 has a positive potential with respect to ground, which potential is equal to the distance C in Figure 2. Let it be assumed that an alternating signal potential is impressed on the ampliher input terminals 31 and 313 of a value such that the amplified signal appearing between the left-hand terminal of resistance 61 and ground is represented by the alternating wave D in Figure 2. It may be seen that the maximum value of the signal wave D is less than the positive bias potential C. Therefore, the signal wave D, superimposed upon the positive bias voltage C, is never effective to overcome the positive bias voltage so as to render the input voltage of the stage |2| negative. The output current of the triode |611 therefore remains at the same substantially constant value throughout the interval E, during which the signal voitage D is maintained. Since there is no change in the output current flowing in triode |60, no signal is transmitted through the blocking condenser 2|| to the control electrodes |85 and |92 of the final power amplication stage |22. The bias voltage established on condenser 255 by means of a rectifier circuit including triode |24 has a polarity such to maintain the control electrodes |85 and |52 negative with respect to their corresponding cathodes, as indicated by the legend in the drawing. The tap 25]!5 is so adjusted with respect to resistance 262 that the potential across condenser M15 is just sufficient to bias both the centrol electrodes |85 and |92 to cutoff.

Under these conditions, when no signal is transmitted through the blocking condenser 2| the relays tti and 81 both remain inactive.

rihe operation of the amplifier circuit will now ered under the conditions which obtain when the incoming signal applied to the second stage il has a maximum value greater than the magnitude of the positivev biasing voltage C. Conditions such as these are illustrated in Figure 2 as occurring during the interval indicated at G, In order to simplify this discussion of the operation of this circuit, it will first be described as though the feedback circuit including resistance li and condensers |5| and |16 were entirely absent.

Let it be presumed then that the alternating input signal superimposed on the positive bias voltage C has a value such as that indicated by the full line F in Figure It Will be noted that the negative peaks of the alternating signal F completely overcome the positive biasing voltage C and render the input voltage of the stage |2| periodically negative. It may readily be seen by projecting the points on the input signal F upward to the overall input voltage-output current characteristic A-B, and then projecting them to the right along a suitable time axis in a well known manner, that the input voltage represented by the positive biasing voltage C plus the alternating signal F produces in the output circuit of the triode i6() a series of negative impulses H. These negative current impulses produce corresponding positive impulses in the voltage between the anode |6| and the cathode |63. These positive voltage impulses are transmitted through the blocking condenser 2|| and applied trzthe grids |85 and |92 of the tetrodes |8| and l The time phase of the impulses is determined by the time phase of the alternating signal F,

which in the system shown depends upon the direction of unbalance of the bridge circuit connected to the amplifier input terminals 31 and 38. Since, in the system shown, the bridge circuit is supplied with electrical energy from secondary Winding Hl on the sarne transformer IIB with the secondary winding IIS Which supplies the output circuit or the tetrodes I8I and I82, it should be apparent that the positive voltage impulses appearing on the grids H35 and I 92 Will be in phase with the anode potential impressed upon one or the other of the tetrodes I8I and E32, depending upon the direction of unbalance of the bridge circuit.

Suppose for erarnple, that the positive voltage waves are applied to the grid l85 so as to make 'that grid more positive during the same half cycle that the anode 233 is positive with respect to the cathode it. Corresponding current irnpulses will then appear in the output circuit of tetrode Il. Each impulse energizes the relay winding t3 and also charges the condenser 222. During the alternate half cycles when no in pulse being transmitted by the tetrode I8I, the condenser discharges through the Winding thereby maint'.V ining it energized. Energizanon or" rela57 Winding 33 operates through servomotor in the manner previously described to ebalance the bridge circuit and reduce the niagnitude of the incoming signal potential to a value than that of the positive bias voltage C. the incoming signal potential has been so l tie relay winding 93 is no longer enerd and the servcinotor is stopped.

I* should be noted that, because of the action oi the positive bias voltage C, the negative peaks of the input voltage and hence the negative peaks lli of the output current are somewhat less than a halt cycle in length. Because of this limitation of 'the length of the impulses, the present ampliller circuit is not critical with respect to the phase of the control electrode potential in the inal stage compared 'with the anode-cathode potential in that stage. It has been found that in any arogller circuit, particularly one wherein e. plurality of stages are cascaded, a small but nevertheless appreciable phase shift taires place between the signal impressed on the input cirt and the signal produced at the output cir cuit. of this phase shift, a signal of a lull cycle duration appearing on the control electrodes and it? might overlap both half cycles of the Wave during which the anodes 83 2" alternately positive. lf the duration of the signal impulses reaching the final stage is li "ed to less than one-half cycle, as in the p; Vr.lent amplifier, considerable latitude as to shift or" the signal from its normal phase positior in either direction is permissible Without c? erroneous operation of the two relays, which are to be selectively controlled in accordance With the phase of the signal.

he possibility of erroneous energisation of the relays tl due to phase shift ci the signals is urther reduced in the present amplier by the negative hie-s on the iinal stage. The control eee-.tively an appreciable diilerential between the bias potential and the minicontrol electrode potential necessary to uce an output current large enough to ener- -Ue relays. The phase of the signal applied to the control electrodes oi the nal stage may nerelore shift by an amount corresponding to time required for the signal to increase to a value corresponding to that differential without causing erroneous operation of the relays.

The operation of the feedback circuit from the unal stage 22 to the second stage Iii, and its effect upon the operation of the amplier as a whole, will novv be considered. Referring to Figure l, it will be seen that when the tetrocles I8! and $82 are both non-conductive, no current flows through the resistance itil) and hence no potential difference is maintained across its terrninals. If the control electrodes 85 and I92 are not biased to cut-off, a small current flow continuously takes place through the resistance iiiil, and a small potential drop is maintained across its terminals. Since the operation of the feedback circuit the same, regardless of whether the normal potential across the resistance itil is assumed to be zero or some positive value, the operation o1" this circuit will be confor the present as though the normal potential across resistance i5@ were zero, in order that this explanation may be simplified as much as possible. By the normal potential across the resistance 55E is meant the potential which exists thereacross when no signal is impressed through the blocking condenser 12H to the control electrodes and ISZ of the unal stage l2. It is believed that il the operation of this circuit is explained when the normal potential across resistance lii is aero, that its operation under other normal potential conditions will be apparent to those slilled in the art.

Referring to 3, it will be seen that when no potential drop exists across resistance i553, that the potential across condenser lit is equal to the potential between tap ils and ground, less the potential drop due to current iiow through the resistance lli?. This potential across condenser il is substantially con tant, and hence has no effect on the input circuit of the stage i2 I.

Let it now be assumed that one of the tetrodes 3i or 32 becomes conductive, thereby establishing a potential drop across resistance 15e. This potential drop has a polarity such that the upper' terminal of resistance itil, as it appears in Figure 3, is positive with respect to the lower terminal. rEhe condenser l is provided to receive a charge during the half cycles when current is flowing through one of the tetrooles in the ilnal circuit, and to discharge through the resistance i513 during the half cycles when no current is -Flowing, so as to maintain the potential drop across resistance F513 substantially constant as long signals are applied to the control electrodes of the nal stage i222.

Referring to Figure 3, it may be seen that the potentials across resistance Ulli and resistance E59 are effectively in series in a loop circuit which may be traced from tap llt through resistance M5, ground connections itt and Isl, conductor U48, resistance ici), conductor Vil, condenser Ile, conductor Ili, rheostat resistance i'lil and con denser IlI in parallel, and the conductor H2 to tap |13. Considering this loop circuit, it may be seen that as the potential drop across resistance |50 increases from zero, the potential applied to the terminals of condenser l'il likewise increases. A charging current the eor flows through the condenser llt and the direction or ilovv of this charging current is from tap llt through resistance llt and conductor lii to condenser Ile. This charging current produces an additional potential drop across resistance il'fl which makes its lower terminal more negative. The variation of the input voltage applied to stage lill as a result o1" this charging current is illustrated in Figure 2 by the curve d".

This additional negative potential applied to the input circuit of stage |2| tends to increase the magnitude of the voltage pulses in the output circuit of that stage. These increased output voltage pulses are in turn reflected in an increased current flow through the resistance |50, with a resulting increase in the charging current flow through resistance |10, thereby making the input voltage of the stage |2| still more negative. The action of the feedback circuit is therefore cumulative but continues so only for av short time, because of the tendency of the tetrodes |8| and |82 of the final stage to become saturated. As the current flow through the resistance |50 approaches its limiting value, which is determined by the saturation ci' the tetrode through Which the current is flowing, the charging current flowing through the resistance |10 begins to decrease in value. This decrease in the potential drop across resistance lill allows the input voltage of stage |2| to become'more positive, thereby decreasing the magnitude of the voltage pulses in the output circuit of stage |2I. A corresponding decrease takes place in the potential drop across resistance |59. As soon as the potential drop across resistance begins to decrease, the potential applied to the plates of condenser |16 is decreased, and the condenser therefore begins to discharge. A` discharging current then iiows in a direction from condenser .Y

output circuit of the stage I2! are thereby further decreased. It may therefore be seen that the effect of the feedback circuit is now becoming cumulative in the opposite direction.

The net eiect oi' the feedback circuit isto supply to the input circuit of stage |2| an alternating potential such as is represented by the curve J in Figure 2'. Although this curve is shown as substantially a sine wave, it will be readily understood that the shape of the wave may substantially depart from a true sine without inte'- ference with the operation of the amplifier circuit. This feedback signal J when added tothe input signal F and the positive bias voltage C produces a net input voltage for the amplifier stage |2| which is of the character indicated by the curve K shown in dotted lines in Figure 2^.

It may therefore be seen that when an alternating signal oi constant amplitude is impressed through the blocking condenser to the input circuit of stage 12|, the feedback circuit operates to alternately increase and decrease' the pulsations Which are passed through thesecond stage |2| and impressed on the control electrodes |85 andv |92 of the ilnalv stage |22. This characteristie is of great value when the alternating input signals are just suniciently large to cause current impulses in the output of stage IZI, but not large enough to produce voltage impulses on the grids |85 and |92 of the final stage suiiicient to overcome the negative bias of that stage and cause operation of the relay. Under such conditions, the feedback circuit comes into play and causes a periodical increase inthe magnitude of the current pulses so that the relay is periodical-ly l. 6 operated, thereby causing the servomotor en to intermittently move the control device along.

For example, in the operation as illustrated in Figure 2, let itv bev assumed that a current irnpulse of an amplitude greater than the distance between the line B and the line' M is necessary to operate upon condenser 2H il the negative bias of the final stage |22 is to be overcome so as to cause operation oi one of the relays. Under such conditions, an alternating input signal of constant amplitude, such as the curve F, would produce current impulses H in the output circuit of stage |2I, which would not cause operation oi arrelay if the feedback circuit were not provided. The feedback circuit comes into play, however. and transforms the input potential ci the stage |2| from the value indicated by the curve F to the value indicated by the curve K. Therefore, the current pulsations in the output of stage |2| are of a magnitude indicated by the dotted lines L in Figure 2. It will be seen that the amplitude of the pulsations L substantially exceeds the distance between the line and the line M, thereby causing energisation of one oi the relays. Because of the operation of the condenser in parallel with the particular relay which is energized, the relay remains energized over the period of at least three cycles because oi the saturation characteristics of the tetrodes during which the impulses L are greater than the distance between the lines B and M.

As a result of the action oli' the feedback circuit, as soon as the amplitude ci lthe alternating input signal exceeds the positive voltage C, a particular relay to be selected in accordance with the phase of the signal ene sized for spaced periods of time. As the maghi'- de of the alternating input signal increases, the frequency of these spaced periods of time remains the saine, but the duration of each period of en-ergisation increases, so that the average rate or' operation of the servomotor 5G is increased in accordance with the magnitude of the input signal. It may also be seen that as the magnitude of the alternating input signal continues to increase, it will eventually reach such a value that the feedback signal J will not be great enough to overcome it and periodically deenergize the relay. When such a value of alternating input s'gnal is reached, the relay selected is continuously energized.

By adjustment of the tap |13 alo g the resistance |45, the positive bias voltage at C in Figure 2 may be adjusted to any desired value. This positive bias voltage determines the minnnum alternating signal passing through blocking condenser 2| which. causes operation of the relay. In other words, the adjustment of tap |13 along resistance M5 regulates the sensitivity of the control system. Adjustment of rheostat resistance |18 varies the amplitude of the potential drop due to the charging and discharging current of the condenser lle, and hence varies the amplitude of the feedback signal applied to the input of sta-ge |2I. Ey adjusting the amplitude of this feedback signal, the range of values of alternating input signal which cause intermittent operation of the relays may be established In other words, while adjustment oi the tap lltl along resistance |45 regulates the value of input signal at which the relay starts to be intermittently operated, adjustment of rheostat |15 determines the value of the input signal at which mtermittent operation of the relay ceases and continuous operation begins. The rheostat |10 17 should not be adjusted to make the amplitude of the feedback voltage J greater than the magnitude of the positive biasing potential C, or a condition might be obtained wherein both relay Would be operated simultaneously upon the occurrence of a large alternating input signal.

Adjustment of rheostat llt also changes the time constant of the feedback circuit to some extent and hence affects the frequency of the feedback signal J. The particular frequency chosen is unimportant, however, as long as it is suilciently high to permit the desired sensitivity of the system.

It may therefore be seen that when the amplier 40 is used in conjunction with the power transmission unit that the power output shaft of the transmission unit is operated at an average rate which varies in accordance with the magnitude of the signal impressed on the amplifier input terminal.

The reason for operating the rudder at a rate proportional to the resulting of the deviation and the rate of deviation is that most aircraft have an inherent tendency to huntf or oscillate, about a vertical axis. In other Words, the aircraft tends to change its direction alternately to opposite sides of the course which it is desired to maintain. It has been found that this tendency may be overcome by introducing into the control system a signal proportional to the rate of deviation of the aircraft from its desired course. On the other hand, there is substantially no tendency of the usual aircraft to hunt about any horizontal axis. If such a tendency were found, it might be overcome in a similar manner.

While I have shown and described a preferred embodiment, other modifications thereof Will readily occur to those skilled in the art, and I therefore wish my invention to be limited only by the scope of the appended claims.

I claim as my invention:

1. An electrical circuit, comprising in combination, an electrical discharge device having an anode, a cathode, and a control electrode, an output circuit for said device including said anode and cathode, a fixed resistance, an input circuit for said device including said resistance, said control electrode and said cathode in a series branch thereof, means, connected to said series branch, for applying a biasing potential across said series branch of a polarity such that said control electrode is positive with respect to said cathode, the potential difference developed across said resistance upon a flow of current through said series branch being effective to limit the potential of said control electrode to a value only slightly positive with respect to said cathode, means, connected to said series branch, for applying a first variable signal potential across said series branch, said biasing potential applying means cooperating with said resistance to prevent any substantial variation of the current in said output circuit except when said signal potential is opposite said biasing potential in polarity and greater than said biasing potential in magnitude, and means, connected to said series branch, for impressing across said series branch a second variable signal potential variable in magnitude dependent upon the value of said first signal potential and periodically varying With an amplitude less than the magnitude of said biasing potential, so as to periodically vary the magnitude of the variations in said output current caused by said first signal.

2. An electrical circuit, comprising in combination, an electrical discharge device having an Si il anode, a cathode, and a control electrode, an output circuit for said device including said anode and cathode, a aed resistance, an input circuit for said device including said resistance, said control electrode and said cathode in a series branch thereof, means, connected to said series branch, for applying a biasing potential across said series branch of a polarity such that said control electrode is positive with respect to said cathode, the potential difference developed across said resistance upon a flow of current through said series branch being effective to limit the potential of said control electrode to a value only slightly positive with respect to said cathode, means, connected to said series branch, for applying a first periodically varying signal potential across said series branch, said biasing potential applying means cooperating with said resistance to prevent any substantial variation of the current in said output circuit except when said signal potential is opposite b :s ig potential in polarity and greater than said biasing potential in magnitude, a control device connected in said output circuit and operated in response to variations in said output current in excess of a predetermined value, and means, connected to said series branch, for impressing across s ries branch a second periodically varying' signal potential variable in magnitude dependent upon the value of said rst signal potential, said second potential being eective when first signal potential has, a constant amplitude greater than said biasing potential, to periodically vary the amplitude of the variations of said output current.

Control apparatus, comprising in combination, an electrical control device operative when supplied With current in excess of a predetermined value, means for producing an alternating signal potential of amplitude variable in accordance with a condition indicative of the need for operation of said control device, amplifier means connected to said signal potential producing means and responsive to said signal potential and effective when said signal potential exceeds a minimum value to produce an output current corresponding to the excess said signal potential over said minimum value, means connecting said control device to said amplier means so that the output current is operative upon said control device and means associated with said amplifier means and controlled by said signal potential for applying to said amplifier means a voltage variable in magnitude dependent upon the value of said signal potential for periodically varying the output current thereof so as to cause at least periodical operation of said control device Whenever said potential exceeds said minimum value.

4. Control apparatus, comprising in combination, a current responsive electrical control device operable betvveen first and second states of activity in accordance with the magnitude of said current and having an appreciable differential between the current values which cause operation of said control device between said states of activity in opposite senses, voltage producing means for producing an alternating signal potential of amplitude variable in accordance with a condition indicative of the need for operation of said control device, ainpiier means connected to said voltage producing means and responsive to said signal potential and effective when said potential exceeds a minimum value to produce an output current corresponding to the excess of said signal over said minimum value, means associated with said amplifier means and controlled by said signal acordaropotentialv for applying. to said amplier; means aA voltagevvaria'ble in magnitude dependent upon the value of said signal potential for periodically vary ing. the output current thereof so as to cause at least periodical operation` of said control device whenever said potential exceeds said minimum value, and means connected to said amplier means for varying said minimum value so as to control the range. of values of said condition wherein operation of said control device is effected.

5. In combination: an electronic ampliner having*` atleast one preliminary voltage amplification stage and a final power amplification stagey each stage having an input circuit and an output circuit; a connection between a source of variable electrical potential and the input circuit of the rst. amplifier stage; a connection between the output circuit of each preliminary amplifier stage and the input circuit of each succeeding amplifierv stage; and feedback circuit means connected between the output circuit of said final stage and the. input circuit of said first stage for impressing a feedba .i potential from the output circuit ofV said' final stage on the input circuit of said one stage and operating to cause intermittent pulsations in said final stage output circuit only when signals of small magnitude are impressed on said first stage input circuit.

6. In confibination:A an electronic ampli-ner having at least one preliminary voltage amplification stage and a final power amplification stage, each stagehaving an input circuit and an output circuit; a connection between a source of variable electrical potential and the input circuit of the first amplifier stage; means connecting the output circuit of each preliminary stage to the input circuit ofV the succeedingA stage; means connected to the input circuit of one of said preliminary stages for applying a positive bias potential to said input circuit; andv feedback circuit means, connected between the output circuity of the final stage and the input circuit of said one stage, for impressing a feedback potential from the output circuit of said final stage on the input circuit of said one stage, said positive bias potential, and said feedback potential interacting to cause intermittent pulsations in said final stage output circuit only when signals of small magnitude are impressed on said rst stage input circuit.

7. In combination: an electronic amplifier having' at least one preliminary voltage amplification stage and a final power amplification stage, each stage having an input circuit and' an output circuit; a connection between a source of' variable electrical' potential and the input circuit of the first amplifier stage; means connecting the output circuit of, each preliminary stage to the input circuit of the succeeding stage; means connected to the input circuit of one of said preliminary stagesfor applying a positive bias potential to said input circuit, feedback circuit means, including an impedance means, connected between the output circuit cf the final stage and the input circuit of. said one stage, for impressing a feedback potential from the output circuit of the final stage on the input circuit of said one stage, said positive bias potential and said feedback potential. interacting to cause intermittent pulsa tions in said final stage output circuit only when a signal potential of small magnitude is impressed on said first stage input circuit; and means connected to said impedance means for adjusting said impedance means so as to vary the` range of magnitudes of` said. signal potential wherein said intermittent pulsationsare effected.

8. An electrical circuit, comprising in combination: a first electrical discharge device having an anode, a cathode and a controlelectrode; a. fixed resistance; an input circuit for said first device includingv said resistance, said control electrode and said cathode in a series branch thereof; an outputv circuit for said first device including said anode and said cathode; a second and a third electrical discharge device each having an anode, a cathode and a control electrode; means electrically connecting said control electrodes o1 said second and third devices together; means connectng said cathodes of said second and third devices together so that said second and third' devices have a common input circuit; a fixed impedance; means electrically connecting said output circuit of said first device to said common input circuit of said second and third devicesthrough said fixed impedance, a first and a second control device; an output circuit for said second discharge device including its anode and cathode and an output circuit for said third discharge device including its anode and cathode, said output circuits having a common portion including their cathodes; means coupling'each of said control devices to one of said output circuits; means connecting said output circuits to a source of alternating electrical energy sothat the potentials applied to said output circuits are in phase opposition; means connected to the series branch in the input circuit of said first discharge device for applying a bias potential across said series branch of a polarity such that said control electrode is positive with respect to said cathode, the potential difference developed across said resistance upon a now of current through said series branch being effective to limit the potential of said control electrode to a valuel only slightly positive with respect to said cathode; means connected to said series branch for applying a rst variable signal. potential across said series branch, said `biasing potential applying means cooperating with said resistance to prevent any substantial variation of the current from the anode of said first discharge` device except when said signal potential is opposite said biasing potential in polarity and greater than said biasing potential in magnitude; and. feedback circuit means connected between the. common portion of the output circuits of said. second and third discharge devices and the input circuit of said first discharge device for connecting a feedback potential from the common portion of theV output circuits of said second and third discharge devices to the input circuit of said; first discharge device, said positive bias potential, said signal potential and said feedback potential interacting to cause intermittent pulsations in said second and said third discharge device output circuits when signals of small magnitude are impressed on said rst discharge device input circui 9. An electrical circuit, comprising in combination: a first electrical discharge device having an. anode; a cathode and a control electrode; a resistance; an input circuit for said' first device including said resistance, said control electrode and said cathode in a first series branch thereof; an output circuit for said first device including said anode and said cathode; a second electrical discharge device having an anode, a cathode and a control electrode; a fixed impedance; an input circuit for said' second device including said im,- pedance, said control electrode and' said cathode in a second series branch thereoi; means electrically connecting said output circuit of said nrst device to said second series branch; a load device; an output circuit for said second discharge device including its anode and cathode; means coupling said load device to said output circuit; means connecting said output circuit to a source of alternating electrical. energy; means connected to said first series branch for applying a bias potential across said first series branch of a polarity such that said control electrode is positive with respect to said cathode, the potential difference developed across said resistance upon a ilow of current through said rst series branch being effective to limit the potential or said control electrode to a value only slightly positive with respect to said cathode; means connected to said first series branch for applying a rst variable signal potential across said rst series branch, said biasing potential applying means cooperating with said resistance to prevent any substantial variation of the current from the anode of said iirst discharge device except when said signal potential isopposite said biasing potential in polarity and greater than said biasing potential in magnitude; and feedback circuit means connected between the output circuit of said second discharge device and the input circuit of said nrst discharge device for connecting a feedback potential from the output circuit of said second discharge device to the input circuit of said rst discharge device, said positive bias potential, said signal potential and said feedback potential interacting to cause intermittent pulsations in said second discharge device output circuit when signals of small magnitude are impressed on said nrst discharge den vice input circuit.

10. In combination: means producing a signal voltage; amplifier means connected to said signal voltage producing means and operative to produce an output current when the signal voltage exceeds a minimum magnitude; a control device so connected to said amplier means as to become operative upon the output current exceeding a minimum value; and means independent of said control device connected to said amplifier means and dependent upon the output current, when the signal voltage is greater than a minimum magnitude and less than a maximum magnitude, to produce a periodically varying voltage which .interacts with the signal voltage to produce an effective periodically varying voltage of such magnitude as to produce an output current which causes intermittent operation of said control device.

11. In combination: means producing a signal voltage; amplifier means connected to said signal. voltage producing means and operative to pro-- duce an output current when the signal voltage exceeds a minimum magnitude; a control device so connected to said amplifier means as to become operative upon the output current exceeding a minimum value; and means independent of said control device connected to an intermediate por tion of said amplifier means and effective when the output current is greater than a predetermined magnitude to cause a variable voltage to be applied to said intermediate portion of said amplifier means in such a manner as to cause said amplifier means to produce an output current which causes at least intermittent operation of said control device when the signal voltage is above a iirst value and below a second higher value.

12. In combination: means producing a signal voltage; ampliiier means connected to said signal voltage producing means and operative to produce an output current when the signal voltage exceeds a minimum magnitude; a relay so connected to said ampliiier means as to become operative upon the output current exceeding a minimum value; and means independent of said relay connected to an intermediate portion of said amplier means and effective when the outA put current is greater than a predetermined magnitude to cause a variable voltage to be applied to said intermediate portion of said amplifier means in such a manner as to cause said ampli iier means to produce an output current which causes at least intermittent operation of said relay when the signal voltage is above a first value.

13. In a control circuit, two vacuum tubes, means for operating said tubes, a relay in the output circuit of each of said tubes, a pulsating voltage source common to the cathodes of both of said vacuum tubes, one of said tubes having a signal impressed on its input circuit, said pulsating voltage source causing intermittent operation of one of said relays upon deviation of said signal from a desired value, a power means comprising a motor responsive to said relay operation to correct said signal to said desired value.

14. In a device of the character described, the combination which includes a pair of discharge devices having a cathode, anode and grid electrodes, a iirst source of alternating current potentials comprising a center tapped transformer winding having the center tap connected to said cathode electrodes and the end terminals connected to said anode electrodes of said devices, a second source of alternating current potentials connected between the grid and cathode electrodes of said devices, said connections being such that in one of said devices the cathode-grid voltage is in phase with the cathode-plate voltage and in the other of said devices the corresponding voltages are in phase opposition whereby one or the other of said devices may become conductive depending on the phase of the potential applied by said second source, output means in circuit with said anode electrodes and responding in a rst mode to current nowing in one of said devices and in a second mode to current flowing in the other of said devices, a rectifier and an impedance connected in series between said center tap and one of said end terminals for developing a rectiied voltage, said impedance being connected to said cathode electrodes, and means for applying at least a portion of said rectified voltage developed across said impedance between said grid and cathode electrodes of both of said pair of devices.

WILLIS H. GILLE'.

References Cited in the file of this patent UNITED'STATES PATENTS Number Name Date 1,858,425 Wittkuhns May 17, 1932 1,931,660 Kautter Oct. 24, 1933 1,956,711 Bellescize May 1, 1934 1,966,607 Brown July 17, 1934 2,175,920 Schnarz Oct. 10, 1939 2,175,921 Schnarz Oct. 10, 1939 2,316,875 Laboulais Apr, 20, 1943 2,442,238 Haungs May 25, 1948 2,425,733 Gille Aug. 19, 1947 2,476,849 Ergen July 19, 1949 

